A mechanical structure for supporting a pair of highly directional systems (e.g., optical telescopes, navigational devices, or high-gain microwave antennas) at a specified separation from each other is ordinarily not sufficiently rigid to maintain perfect correlation between the orientations of the two directional systems relative to one another. Unintended changes are apt to occur in the relative orientations of the pair of spatially separated directional systems mounted on the common supporting structure, because of distortions occurring in the supporting structure due to thermal, aerodynamic, vibrational and/or gravitational effects, and/or due to a phenomenon known as "material creep".
As a consequence of such unintended changes in the relative orientations, data collected or generated by the directional systems are likely to be faulty--unless mechanical or electronic compensation is introduced to nullify the effects of the changes in the relative orientations, or unless the data are adjusted to account for the changes in the relative orientations. Techniques for mechanically and/or electronically compensating for changes in the relative orientations of a pair of directional systems, as well as techniques for adjusting data to account for such changes, require that the angular coordinates of one of the directional systems be precisely correlated in three dimensions with the angular coordinates of the other of the directional systems with a high degree of accuracy.
In recent years, the measurement of distortions in mechanical supporting structures has become an increasingly critical problem in a wide variety of applications, particularly as systems mounted on mechanical supporting structures have become more highly directional. Distortions in mechanical supporting structures can generally be attributed to three categories of effects, viz.:
1) one-time effects (e.g., effects associated with such one-time events as deployment, or curing, or--for extraterrestrial space applications--"gravity release"); PA1 2) low-frequency effects (e.g., thermal or tidal effects); and PA1 3) high-frequency effects (e.g., vibrational transients).
A need has been recognized in the prior art for a reliable technique of general applicability for measuring distortions occurring in mechanical supporting structures, which technique can be adapted to the requirements of specific applications by relatively simple adjustments of conventional parameters in accordance with well-known physical principles.
In an article entitled "Three Axis Angular Monitoring System for the Magsat Mission" published in SPIE Proceedings, Vol. 251, July 1980, by P. W. Collyer and F. W. Shenkel, a concept was described for obtaining measurements of displacements in three angular degrees of freedom due to distortions occurring in a satellite designed for the Magsat Mission. These distortions occurring in the satellite, which are characterized as roll-, pitch- and yaw-distortions, are measured with respect to a coordinate system that is internal to the satellite, and are independent of any roll, pitch or yaw that the satellite as a whole might undergo relative to an external coordinate system.
According to the concept proposed by Messrs. Collyer and Shenkel for measuring distortions occurring in the Magsat Mission satellite, a "pitch/yaw sensing head" is mounted at a first end (called the active end) of an elongate supporting structure, and a flat mirror is mounted at the other end (called the passive end) of the elongate supporting structure. The active end of the supporting structure is secured to one side of the satellite, and the passive end thereof is secured to an opposite side of the satellite. The pitch/yaw sensing head comprises a first light source and a first analog area detector. A light beam from the first light source is reflected by the flat mirror so as to form an image of the first light source on the first analog area detector. The position of the image on the first analog area detector provides a measurement of the pitch and yaw attributable to distortions occurring in the supporting structure. In addition, a "twist sensing head" and a first dihedral reflector (i.e., a standard rooftop mirror) are mounted at the active end of the supporting structure, and a cross-oriented second dihedral reflector is mounted at the passive end thereof. The twist sensing head comprises a second light source and a second analog area detector, which are both mounted on one side of the first light source. The first dihedral reflector is mounted on an opposite side of the first light source. A beam of light from the second light source is reflected from the second dihedral reflector to the first dihedral reflector, and then back to the second dihedral reflector for reflection therefrom so as to form an image of the second light source on the second analog area detector. The position of the image on the second analog area detector provides a measurement of the roll attributable to distortions occurring in the supporting structure.
Disadvantages associated with the above-described concept for measuring roll-, pitch- and yaw-distortions include a low data rate (i.e., less than about 5 Hz) inherent in the use of analog area detectors, a significant drift (i.e., about 25 microradians) caused by electronic circuitry for reading the analog area detectors, a restricted dynamic range (i.e., less than about 2 milliradians between points separated by over 10 meters), and relatively poor accuracy (i.e., only about 25 microradian accuracy at best).
Interferometric techniques have also been proposed in the prior art for measuring distortions that occur in a mechanical supporting structure. According to a typical interferometric technique, interferometers (normally about six) using non-parallel laser beams are arranged at specified positions on the supporting structure to enable changes in optical path length from retroreflecting targets mounted at other positions on the supporting structure to be measured by triangulation. In principle, such interferometric measurements can provide sufficient information to determine relative roll-, pitch- and yaw-distortions, as well as to specify relative translational position, between large portions of the supporting structure. However, since triangulation requires that relatively large separations be provided between the interferometers and also that relatively large separations be provided between the retroreflecting targets, it is not generally possible for an interferometric measuring technique to measure very localized distortions in the supporting structure. The accuracy of an interferometric measurement of distortion is highly dependent upon the geometrical arrangement of the interferometers and the retroreflecting targets. In many applications, physical constraints imposed by the environment, or by operating conditions, preclude a geometrical arrangement of the interferometers and retroreflecting targets necessary for obtaining precise interferometric measurements.
To obtain a high degree of accuracy using an interferometric technique for measuring distortions in a mechanical supporting structure, it would ordinarily be necessary to provide a large exclusionary zone (i.e., a zone that is free of components of any kind protruding from a surface) between the interferometers and the retroreflecting targets. Such an exclusionary zone might be difficult or impossible to provide in many applications. Furthermore, to accommodate a reasonable dynamic range of distortions, two-dimensional scanning mirrors and servo-loops might be required for each interferometer to ensure that the individual interferometers track their corresponding targets. Also, it should be noted that interferometers only measure relative linear displacements, and are subject to occasional resets. If any of the laser beams were to fail even momentarily, the accuracy of the measurements of roll, pitch and yaw would be degraded.